U.S. Pat. Nos. 4,257,579 and 4,340,207 describe a heat recovery process and apparatus for recovering heat from waste gases having a temperature of about 500.degree. F. to about 2500.degree. F., when the flow of hot gas is intermittent. Heat transfer salt and/or heat transfer oil provides thermal storage. The stored heat is evenly transferred to other steady processes such as preheating air, generating steam for process use or driving a steam turbine or for process heating.
Steel mill reheat furnaces often use combustion air in waste gas recuperators to attempt to recover the heat from the waste gas. These recuperators are inefficient, and subject to equipment failures. In a typical design, the recuperator should cool the hot gas from 1730.degree. F. to 965.degree. F. by heating air from 70.degree. F. to 1015.degree. F. The actual performance typically cools the hot gas from 1200.degree. F. to 1000.degree. F. by heating air from 70.degree. F. to 688.degree. F. The recuperators are inefficient because they recover less than 50% of the available heat. Moreover, the recuperators are subject to equipment failure because of the high metal temperatures (1300.degree. F. to 1500.degree. F.) and frequent wide swings in temperature (normally 1000.degree. F. to 1800.degree. F. and sometimes 60.degree. F. to 2000.degree. F.).